This application relates to frequency locking of tunable lasers, and more specifically, to techniques for stabilizing the output frequency of a laser by using an optical cavity to provide a frequency error signal and techniques for monitoring a frequency drift between a laser and an optical cavity.
Certain lasers are tunable in frequency to produce a variable output laser frequency in response to a change in one or more laser parameters. For example, the cavity length of a laser may be adjusted to change the output laser frequency. In semiconductor diode lasers, the driving current may be adjusted to tune the output laser frequency.
A control mechanism for adjusting the output laser frequency in a tunable laser may be used to not only tune a laser but also lock a laser at a specified laser frequency. Some laser locking systems use a laser frequency monitoring mechanism to monitor the drift of the output laser frequency with respect to a frequency reference. When the output laser frequency deviates from the specified laser frequency beyond a specified tolerance range, the control mechanism adjusts one or more laser parameters to reduce the frequency deviation and hence locks the laser.
This application describes techniques and systems for monitoring a difference between the frequency of a laser and a resonance of a birefringent optical resonator. A feedback control loop may be used to lock the laser to the resonator, or alternatively, to lock the optical resonator to the laser.
In one embodiment, a laser frequency monitoring system includes an optical resonator having a birefringent medium located inside the resonator. The resonator is positioned to receive a laser beam produced by the laser to reflect a portion of the laser beam as a reflected optical signal and oriented so that said linear polarization of the laser beam is not parallel to either of two principal polarization axes of said birefringent medium. The system also includes an optical element located in an optical path of at least a portion of the reflected optical signal to produce a phase shift of about 90 degrees between polarizations respectively along the two principal polarization axes. An optical polarizer is located in an optical path of at least a portion of a transmitted signal from the optical element to mix two orthogonal polarizations to produce a mixed optical signal. In addition, the system has an optical detector positioned to receive the mixed optical signal to produce a detector signal which has an AC component indicating a frequency change in the laser frequency.
A laser control unit may be coupled to control a laser parameter of the laser according to the AC component to reduce the change in the laser frequency.
Alternatively, the birefringent cavity may be engaged to a tuning element which operates to change an optical path length of the cavity in response to a control signal. The AC component may then be used to tune the cavity to reduce the frequency difference from the laser.